The purposes of pressure medium containers of the type in question (containers for short) are many and various and consist primarily in serving as a receiving reservoir for a fluctuating pressure medium filling level, which can be caused by unavoidable system processes, such as slopping of the pressure medium and by a variable filling level in the brake system (elasticity, brake lining wear process and brake lining replacement process, lining wear compensation process, hydraulic control processes in the electronic brake system). While a pressure medium reservoir with a holding capacity of particularly large dimensions does have functional advantages in the case of large fluctuations in filling level, this generally conflicts with the requirement for a reduced installation space and the rest of the requirement profile (costs etc.) on modern motor vehicles. Consequently, a receiving reservoir for brake fluid is in principle made as small as possible to match the minimum requirement, allowing for an emergency operation reservoir, in order to fully satisfy the requirement profile set. In addition, there is a desire for a simple and automated way of introducing pressure medium under a vacuum in a motor vehicle brake system in a reliable process at the end of the line at a motor vehicle manufacturer. For maintenance operations, it should be possible to change the pressure medium easily and in a reliable process in a specialist workshop. For this purpose, the general practice is to provide a filler opening which can be closed reversibly by means of a closure element and which can accept a coupling piece of a vacuum filling device for filling purposes. Generally, the pressure medium container is assigned an electronic filling level indicator. This serves to indicate to a vehicle driver during operation of the motor vehicle when a pressure medium filling level in the container falls below a minimum permissible pressure medium filling level.
Known pressure medium containers have at least one venting device for atmospheric pressure equalization with the surrounding atmosphere in order to ensure a current atmospheric pressure within the container at all times. A deviation in the pressure within the container from the current atmospheric pressure can lead to unwanted functional impairments of the hydraulic brake system, e.g. rubbing brakes, reduced braking force or contamination with foreign matter.
Moreover, pressure medium containers are generally designed in such a way that an upper edge of the filling opening assumes a horizontal position when the pressure medium container is introduced into its envisaged installation position in the motor vehicle.
Checking of the regulation of a pressure medium quantity, e.g. by replenishment or removal by suction during maintenance work, is generally performed manually, with visual filling level markings (min/max) for the attention of the operative being provided on the pressure medium container.
It has been found that certain, possibly interlinked, boundary conditions can lead to an impermissibly high pressure medium filling level. These include maintenance operations carried out incorrectly, for example, which can cause overfilling because, for example, a maximum marking has not been correctly observed during replenishment of the pressure medium or when changing the pressure medium. Brake lining changes which have been carried out and which, as a rule, require resetting of wheel brake cylinders, can likewise cause this problem unless excess pressure medium has been sucked out of the container. Electronic control operations or a transfer of pressure medium quantities held in other parts of the brake system, e.g. from an actuator, can likewise cause a rising pressure medium liquid level in the pressure medium container, at least for a brief period, and this can lead to an unintended escape of pressure medium.